1. Field of the Invention
The present invention relates to fume exhaust ducts and more particularly to a dual wall duct having a laminated inner wall whose innermost layer incorporates continuous, helically-wound carbon fibers which dissipate and neutralize built-up electrostatic charges resulting from vapors or gases flowing within the duct.
2. Description of the Related Art
Ductwork for corrosive or otherwise dangerous vapor and gas exhaust systems is used extensively in many diverse industries which utilize hazardous chemicals to process raw materials or perform manufacturing procedures, such as the semiconductor industry, the plating industry, and the pharmaceutical industry. Such ductwork also is required in the many research and development laboratories which use highly reactive, toxic or otherwise hazardous chemicals in conducting experiments. Consequently, vapors from such chemicals must be exhausted through leak-proof air ducts to safely remove them from work areas. Installations can be very large, consisting of many thousands of feet of ductwork which may be manifolded and connected to multiple exhaust fans. Because of the wide diversity of chemicals used in industrial and research applications, it is extremely difficult to provide a single material for fabricating ductwork which can withstand all the chemicals to which duct interiors may be exposed. Materials which have been used heretofore to fabricate fume exhaust ductwork include black steel, galvanized steel and stainless steel, as well as plastic materials such as polyvinylchloride, polypropylene, coated materials, and fiberglass reinforced plastics (FRP""s). Over the past forty years the trend in materials has been away from metals and coated metals and toward the use of plastics, particularly FRP""s.
Various types of resins have been used in manufacturing FRP""s. Some are resistant to certain families of chemicals, but no single resin can resist all the vapors used in industries such as semiconductor manufacturing and electroplating. Another problem is providing adequate resistance to fire. Unlike metallic ducts, plastic ducts exhausting chemicals which can react exothermically with themselves or with duct surfaces are at risk to combustion. Plastics typically burn rapidly and generate much smoke, creating hazards of their own. A third problem in using plastic is the possibility of static electrical charge building up on a duct""s interior surface. Electrostatic charge is usually generated by frictional processes during contact and separation of materials. So-called xe2x80x9ctriboelectric chargingxe2x80x9d can occur even when dry media such as air or air laden with dust particles flow through ungrounded plastic pipes. Because plastic is an insulator, a considerable amount of charge can be generated on a duct""s inner surface. An insulative material does not readily allow the flow of electrons, so both positive and negative charges can reside on the surface at the same time, although at different sites. If the voltage at a site builds up sufficiently, an electrostatic discharge will occur creating an arc which, depending on the characteristics of the ambient vapor, can trigger an explosion or fire. Perhaps the worst cases when exhausting hazardous vapors are those containing halogens such as fluorine or chlorine and those containing metallic vapors such as cesium or potassium. Such vapors are easily ionized as well as highly corrosive and/or toxic.
U.S. Pat. Nos. 5,298,299 (xe2x80x9c""299xe2x80x9d) and 5,383,994 (xe2x80x9c""994xe2x80x9d) to L. E. Shea, which are incorporated in their entirety herein by reference, are directed to, respectively, a composite fume exhaust duct having both good chemical resistance and good fire resistance properties, and a method for making the dual-laminate duct. Ducts made in accordance with the invention described in these patents are generally tubular with a diametral size in the range of 2 inches to 84 inches, and have an inner laminate portion of chemically resistant material covered by and integral with an outer laminate portion of fire retardant material. The inner laminate is comprised of material such as fiberglass which is saturated with a chemically resistant resin such as a halogenated vinyl ester. The outer laminate which covers the inner laminate is similarly comprised of fabric or fiberglass material which is combined with a resorcinol or phenol/resorcinol type fire-retardant resin. The duct is formed by first coating a Mylar wrapped mandrel with the chemically resistant resin and then wrapping the mandrel with successive layers of fabric material saturated with the resin. The outer fire-retardant laminate is then formed directly over the inner laminate by applying successive layers of a suitable fabric material saturated with the fire-retardant resin. The composite structure is then allowed to cure and harden before being removed from the mandrel. U.S. Pat. No. 5,549,949 (xe2x80x9c""949xe2x80x9d) to D. Williams et al., which is incorporated in its entirety herein by reference, is directed to sealant compositions for sealing the circumferential joint between pairs of dual-laminate fume duct sections, and a joint sealing method enabling strong bonding between the sealant and phenolic/glass and vinyl ester surfaces without sanding mating surfaces. Such duct sections and connecting joints do not, however, eliminate or even mitigate the hazard of static charge build-up on the innermost surface of the inner laminate, which typically has a resistivity of the order of 1014-1015 ohms-cm.
Application Ser. No. 09/603,468 (xe2x80x9c""468) is directed to a dual wall fume duct wherein the innermost layer of the laminated inner wall is made of a cured epoxy vinyl ester resin impregnated with chopped carbonized carbon fibers. The inner surface of the duct described therein can withstand degradation from corrosive fumes while also neutralizing triboelectric charges. However, the fabrication method needs to be improved to achieve higher production rate and better quality control of the end product. Applying the carbon fibers to a rotating mandrel coated with liquid resin is a time-consuming manual procedure not amenable to standardization and quality control. Moreover, the ""468 application is not directed to providing high electrical conductivity across connecting joints.
In view of the limitations of the related art, it is a primary object of the present invention to provide an improved self-grounding fume exhaust duct, having an innermost portion withstanding hazardous fumes while dissipating and neutralizing electrostatic charge build-up, fabricated using a substantially automated method.
Another object of the invention is to provide a self-grounding duct fabricated using a production method amenable to standardization and high quality control.
Yet another object of the invention is to provide a self-grounding joint for joining duct sections.
Other objects of the invention will become evident when the following description is considered with the accompanying drawing figures. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawings and description.
These and other objects are achieved by the present invention which in a first aspect provides a self-grounding dual wall duct for transporting corrosive vapors and gases having a laminated inner wall including an innermost layer of cured epoxy vinyl resin in which are embedded contiguous helical segments of electrically conductive continuous ribbons forming a continuous ribbon-layer integrated with the innermost layer inner surface.
In another aspect the invention provides a process for fabricating a self-grounding dual wall laminated duct for transporting corrosive vapors and gases, which includes the steps of: forming a fluidic admixture of an epoxy vinyl ester resin and a curing agent; evenly coating a horizontal mandrel covered with a polyester sheeting with the admixture to form a thin layer; helically winding around the mandrel a band including electrically conductive, continuous ribbons, disposed edge-to-edge, each having continuous longitudinal carbon filaments impregnated with the admixture, so that contiguous helical segments form a continuous first ribbon-layer which is embedded in the admixture layer and integrated with the layer interior surface; helically winding additional band to form a thickness of ribbon-layers; evenly coating a second layer of the admixture; rolling out air trapped in the ribbon-layers; helically winding a layer of mat-type fiberglass, post-wetted with the admixture; and curing the admixture.
The process further includes: forming an admixture of a phenolic impregnating resin and a catalyst therefor; evenly coating the fiberglass outer surface with a thin layer of the admixture; helically winding a band of continuous glass filament ribbons impregnated with the admixture to form an inner glass filament layer; rolling out air trapped in the inner layer; helically winding additional band to form an outer glass layer; rolling out air trapped in the outer glass layer; helically winding a layer of glass veil, wetted out with the admixture; and curing the admixture.
A more complete understanding of the present invention and other objects, aspects and advantages thereof will be gained from a consideration of the following description of the preferred embodiment read in conjunction with the accompanying drawings provided herein.